(A) Field of the Invention
The present invention relates in general to a thin film transistor (TFT) array substrate. In particular, the present invention relates to a TFT array substrate with high aperture ratio pixel structure.
(B) Description of Related Art
Currently, liquid crystal display (LCD), having the advantages of low power consumption, being thin type, light weight and having low driving-voltage, has been put to practical use, such as in personal computers, archival treatments, navigation systems, projectors, view finders and portable machines (watches, electronic calculators and televisions). The applied electric field on the LCD changes the alignment of liquid crystals that possess the characteristics of dielectric anisotropy and conducting anisotropy, and then the accompanied optical effects will be transformed into visional variation. A LCD employing a thin film transistor (TFT) as an active device is named TFT-LCD, which is good for low power consumption, is thin type, light weight and has low driving-voltage.
FIG. 1A is a top view showing a pixel structure of conventional TFT-LCD. The scanning lines 110 and the signal lines 120 are arranged in a matrix form. Thin film transistor (TFT) 130 is disposed over the scanning line 110, and a pixel electrode 140 is electrically connected to the TFT 130. A black matrix 150 (as shown in FIG. 1B) has a window 152 pervious to light. FIG. 1B is a sectional view showing the structure along the line A–A′ of FIG. 1A. The scanning line 110 (not shown in FIG. 1B), signal lines 120 and pixel electrode 140 are formed on the lower substrate 102. The black matrix 150 and the window 152 are formed on the upper substrate 104. Because the light passing through the edge of the pixel electrode 140 is shielded by the black matrix 150 to avoid light leakage, the transmittance area I is only just as broad as The window 152. In other words, the aperture ratio is reduced. Therefore, another proposal of improved pixel structure is disclosed.
FIG. 2A is a top view showing another pixel structure of conventional TFT-LCD. Two floating electrodes 160 are disposed on the lower substrate 102 (as shown in FIG. 2B) to shield the leakage light passing through from the edge of the pixel electrode 140 near the signal lines 120. FIG. 2B is a sectional view showing the structure along the line B–B′ of FIG. 2A. Therefore, the window 154 of the black matrix 150 is broader than the window 152 shown in FIG. 1B. In another word, the transmittance area II is bigger than the transmittance area I (as shown in FIG. 1B) so the aperture ratio of pixel structure is improved. Moreover, the aperture ratio of the pixel structure shown in FIG. 2A is increased about 3% comparing to the pixel structure shown in FIG. 1A.
However, the aperture ratio of FIG. 2A is still decreased under a misalignment assembly process. FIG. 3 is a sectional view showing a state of misalignment assembly of conventional TFT-LCD. The upper substrate 104 is shifted along the direction of an arrow 180 under the misalignment assembly process, and the black matrix 150 following the upper substrate 104 is also shifted. Therefore, the transmittance area is decreased from II to II′, and the improved aperture ratio is reduced again. According to actual experiment result, shifting the black matrix 150 1 μm decreases the aperture ratio 1% the aperture ratio 1%. For this reason, if the black matrix 150 is shifted more than 3 μm, the aperture ratio will be reduced more than 3%. Consequently, the improved aperture ratio will be canceled out completely.